Modifier keys are special keys on a computer keyboard that change the function of other keys when pressed together. Common modifier keys include Shift, Ctrl (Control), Alt (Alternate), and Cmd (Command on Mac). For example, holding down the Shift key while pressing a letter key will produce an uppercase letter. These keys are essential for executing various commands and shortcuts. For instance, pressing Ctrl + C copies selected text, while Ctrl + V pastes it. Modifier keys enhance productivity by allowing users to perform multiple actions quickly without needing to navigate through menus.